DTMF is a means of implementing communications between telephone equipment. DTMF utilizes a unique pair of frequencies to represent each of a plurality of digits, and was initially invented as a way to allow an end user to communicate to a central office, using a telephone terminal (the number of a called party). In this initial application, after the central office received the DTMF tones, it could disable or ignore its DTMF detector and use the information (i.e, the called party's telephone number) acquired from the DTMF tones to route the call through the telephone network.
As is well known in the art, DTMF is now commonly used as a means for a user to provide a signal to an apparatus which is performing a voice processing application. For example, DTMF is now used to allow bank customers to request balances, transfer money between accounts, etc. DTMF is used by numerous businesses to allow customers to select purchasing options, etc., and in countless other business applications.
In these newer types of DTMF applications, the properties of the DTMF environment are not as well defined as in the initial application previously set forth. For example, in these new applications, the DTMF detector must be capable of receiving and correctly decoding DTMF tones which have been transmitted (i) from a user equipment to a PBX, (ii) through a central office switch, (iii) over a long distance network, (iv) through another central office switch to a receiving PBX, and (v) finally to a voice processing apparatus.
Furthermore, since the DTMF is now being used after the call is connected, not just for call set up and routing as was previously the case, there will be voice and other signals on the line while the DTMF detector is active. Indeed, voice processing applications generally require the DTMF detector to be capable of recognizing DTMF tones simultaneous with recording a message or playing a message. Nonetheless, the DTMF detector must be capable of interpreting the tones in a reliable manner, even though these tones may be significantly degraded when compared to DTMF tones transmitted from a user equipment to a DTMF detector located at the user equipment's own local central office. If the DTMF detector mistakes an outgoing recording from the voice processing system or an incoming speech signal as a valid DTMF tone, the entire voice processing system may enter some undesired mode, resulting in system failure.
In order to deal with the numerous problems of DTMF detection in a voice processing environment, many techniques have been proposed. See, for example, U.S. Pat. No. 4,599,495 to Richards. Nearly all of these prior art arrangements utilize complex signal processing in the DTMF detector in order to distinguish voice from DTMF tones. These arrangements therefore all require a complex and expensive DTMF detector.
U.S. Pat. No. 4,386,239 to Chien describes, in the background section, an arrangement that utilizes a voice filter to turn off the detector if the energy detected within the voice band is higher than a predetermined level. Such a system suffers from numerous drawbacks. For example, if the energy in the voice band is higher than the predetermined level, it may very well be noisy DTMF tones producing that energy, rather than voice. Moreover, a sudden spike of noise on the line can drive the amplitude of the signal in the voice band above the detection threshold, thereby resulting in shutdown of the DTMF detector. Finally, played back voice messages which are not properly echo cancelled may result in the DTMF detector being turned off.